Increasing emphasis is being placed on the protection of industrial power distribution circuits against the damaging consequences of ground faults. Typically, the magnitude of current flowing through phase to ground faults is too low to produce an early response by conventional overcurrent sensing means incorporated in circuit protective devices such as circuit breakers. The majority of ground faults are initially attended by arcing which in time causes extensive damage at the site of the fault before escalating to fault currents of overcurrent proportions.
To sense the presence of ground faults on an electrical distribution circuit, the conventional approach is to monitor the vectorial sum of the currents flowing in the various distribution circuit conductors pursuant to determining whether or not all of the current flowing to the load via these conductors is returning to the source via these same conductors. If not, then the resulting imbalance must be occasioned by current returning to the source by an unintended circuit path, typically a ground circuit path. To implement this approach, a differential or so-called "zero sequence" current transformer is normally utilized. This transformer includes a core situated in embracing relation with the distribution circuit conductors and on which is wound a multi-turn secondary winding. Should the vectorial sum of the currents flowing in these conductors not equal zero, a fault signal is developed in the secondary winding which is proportional in magnitude to the differential of the vectorial sum from zero. This fault signal is fed to a ground fault signal processor pursuant to initiating the execution of a ground fault trip function by a suitably equipped circuit interrupter.
In the case of high current, polyphase industrial power distribution circuits, the individual circuit conductors are necessarily quite large in cross-section and separated from each other for requisite interphase clearance. Consequently, the core of a differential current transformer must be quite large in order to embrace all of the phase conductors. In the close confines of industrial switchboards, the installation of a differential current transformer presents a considerable packaging problem. This is particularly so in the case of fusible circuit interrupters where space problems are exacerbated by the presence of a plurality of fuses typically of quite large physical size.
It is accordingly an object of the present invention to provide a ground fault circuit interrupter of compact physical size.
A further object of the present invention is to provide a fusible ground fault circuit interrupter of the above character.
Other objects of the present invention will in part be obvious and in part appear hereinafter.